Electrochemical conversion cells, referred to as fuel cells, produce electrical energy by processing first and second reactants, e.g., through oxidation and reduction of hydrogen and oxygen. By way of illustration and not limitation, an example polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) positioned between a pair of catalyst layers and a pair of gas diffusion media (GDM). The GDM can include a gas diffusion layer and a microporous layer (MPL). The cathode electrode layers can be part of the PEM (catalyst-coated membrane (CCM)) or as a layer on the GDM (catalyst coated diffusion media (CCDM)). This assembly is called a unitized electrode assembly (UEA). A cathode plate and an anode plate (or bipolar plates BPP) are positioned at the outermost sides adjacent the gas diffusion media layers and the preceding components are tightly compressed to form a single cell unit.
The UEA also includes a subgasket material which acts to separate the anode and cathode flow streams and provides for electrical insulation between the anode and cathode plates (or BPPs) of the fuel cell. It is desirable for the subgasket film material to extend to or slightly past the plate edges in order to act as an electrical insulator between the anode and cathode plates of a cell. A subgasket that follows a periphery of the anode and cathode plates abuts the MEA. An inner edge of the subgasket defines the active region of the MEA.
The dimensions of the subgasket may be important for providing adequate sealing between the BPPs of a fuel cell stack, and militating against adverse effects of electrolyte membrane expansion and electrolyte membrane shearing under compression of the UEA. A subgasket having a single thickness may affect the fuel cell performance, in that a tenting region may be formed between the diffusion media and the electrolyte membrane. The tenting region is a cavity formed between the electrolyte membrane and the diffusion media, adjacent a lap joint between the subgasket and the membrane. The tenting region is generally triangular cross-sectional shape.
When the subgasket has a single thickness that is too thick, it may be too rigid to optimize the life of the UEA. Expansion of the electrolyte membrane in the tenting region can cause the electrolyte membrane to buckle, and can cause damage to the electrolyte membrane. Additionally, repeated expansion and contraction of the electrolyte membrane may cause excessive wear of the electrolyte membrane along the edge of the subgasket when UEA over-compression is present. Minimization of a total cross sectional area of the tenting region may be desired to militate against excessive wear and premature failure of the electrolyte membrane and/or diffusion media.
To minimize the cross-sectional area of the tenting and/or thin a subgasket, a laser ablation process may be used. In a laser ablation process, the laser can sublimate an unfinished subgasket film to thin the subgasket in the desired areas, forming a finished subgasket. However, laser ablation processes can be quite costly. Alternatively, the unfinished subgasket film can be heated and pressed against a mold surface. However, this process fails to adequately thin a subgasket as there is limited volume for displacing the necessary material to thin a subgasket. Therefore, there is a desire to develop a process that can adequately thin a subgasket and is less costly.
When a subgasket is too thin, it may be mechanically weak and difficult to handle. The subgasket may be subjected to excessive deflection caused by a flow of gaseous reactants in the feed region where the diffusion media is not present. When excessively deflected, the subgasket may enter and restrict gaseous reactant communication to and from the active areas of the bipolar plate. In addition, when a UEA having a thin subgasket is assembled into a fuel cell stack as a discrete component, it can be difficult to obtain good positional alignment of the trimmed UEA profile features to the BPP features in the fuel cell stack assembly. Thus, there is a desire to reduce the thickness of the subgasket material to reduce costs and decrease membrane stress at the subgasket edge where the anode and cathode GDMs overlap the subgasket, while avoiding the issues associated with thin subgaskets.
The subgasket may also contain other components to form a subgasket assembly. Seals may be disposed on the subgasket to keep reactant gases (e.g., hydrogen and oxygen) within their respective regions thereby preventing them from escaping the fuel cell. Reinforcing feed shims may also be used to provide structural support for the subgasket over the channel features in the feed region of the plate. An insulator may also be added along the perimeter of the subgasket to further insulate and provide protection from short-circuiting between the BPPs. In assembling the subgasket assembly, it can be difficult to assemble each part (e.g., seal, shim, insulator, etc.) as each introduces thickness considerations and placement challenges. Thus, it may be desirable to keep the subgasket thin in the areas where the diffusion media overlaps it to minimize the tenting region area, and thick in the feed region area so that the subgasket is structurally sound and can avoid, for e.g., excessive deflection.
In view of the foregoing, it would be desirable to develop a subgasket, an integrated subgasket assembly, a UEA, and an integrated fuel cell assembly, along with methods of production thereof, in order to prolong the operating life of a UEA and fuel cell, while minimizing manufacturing costs.